1. Field of the invention
This invention relates to a gradual-releasing, i.e., sustained release, capsule containing a hydrophobic oil in the core and to a method for manufacturing the same.
2. Description of the Related Art
A capsule having a uniform grain size and which contains, in a curable polymer, a hydrophobic oil which is introduced using a multiple nozzle, e.g. a double nozzle, triple nozzle, and its production method called "insolubilizing method" (or a nozzle method) have been known for long. The production of a capsule by the insolubilizing method is described, for example, in the reference "Microcapsule" edited by Tamotsu Kondo, Japan Standards Association, p. 248, and in Japanese Patent Application Laid-Open (JP-A) Nos. S44-11914 and H65-9178.
A method called "in situ polymerization method" is known in which a mixture consisting of a reactive monomer, hydrophobic oil, and polymerization initiator is dispersed in water using a surfactant and this emulsion solution is heated or irradiated with light to polymerize the reactive monomer in the emulsion thereby producing a capsule. This method is also described, for example, in the reference "Microcapsule" edited by Tamotsu Kondo, Japan Standards Association, p. 24.
The insolubilizing method makes it possible to prepare a uniform capsule with a relatively large grain size of 0.5 to 5 mm. The in situ polymerization method yields a microcapsule with a grain size of several micrometers to tens of micrometers and is therefore applied to carbonless paper or the like.
The preparation of a multiple capsule which has a relatively large and uniform grain size, contains a hydrophobic oil therein, and is strong and superior in gradual-releasability is, however, difficult as in the past.